1. Leishmaniasis is transmitted by the bite of infected female phlebotomine sand flies. During a blood meal, the sand flies inject metacyclic promastigotes (the infective stage) from their proboscis. 2. Promastigotes are phagocytized by macrophages and other mononuclear phagocytic cells. 3. In these cells, promastigotes transform into amastigotes (the tissue stage). 4. Amastigotes multiply by simple division and infect other mononuclear phagocytic cells. 5–6. During a blood meal on an infected host, sand flies become infected by ingesting macrophages infected with amastigotes. 7. In the midgut of the sand flies, amastigotes transform into promastigotes. 8. There, they multiply, develop, and migrate to the proboscis.
Image from the Centers for Disease Control and Prevention Image Library.